1. Field of the Invention
The present invention relates to lithium secondary batteries and methods of manufacturing the batteries.
2. Description of Related Art
Mobile information terminal devices such as mobile telephones, notebook computers, and PDAs have become smaller and lighter at a rapid pace in recent years. This has led to a demand for higher capacity batteries as the drive power source for the mobile information terminal devices. With their high energy density and high capacity, lithium-ion batteries that perform charge and discharge by transferring lithium ions between the positive and negative electrodes have been widely used as the driving power sources for the mobile information terminal devices.
The mobile information terminal devices tend to have higher power consumption as the functions of the devices, such as moving picture playing functions and gaming functions. It is strongly desired that the lithium-ion batteries that are the drive power source for the devices have further higher capacities and higher performance in order to achieve longer battery life and improved output power. In view of these circumstances, attempts have been made to use a silicon alloy and a tin alloy, which can absorb a large amount of lithium per unit volume, as the negative electrode active material so that the discharge capacity of the lithium secondary battery can be increased.
The lithium secondary battery that uses the above-described negative electrode active material, however, suffers from the expansion and shrinkage of the negative electrode active material that are associated with charge-discharge operations. This results in fractures of the binder resin, which binds the negative electrode active material together inside the negative electrode active material layer and joins the negative electrode active material with the negative electrode current collector, and peeling-off of the negative electrode active material and the negative electrode current collector at the interfaces with the binder resin. Therefore, a problem has been that the current collection structure is broken within the negative electrode, resulting in degradation of electron conductivity inside the negative electrode and poor cycle performance.
In view of the problem, it has been proposed to use a high-strength polymer such as polyimide as the binder of the negative electrode (see, for example, WO 2004/004031).
Even when the just-mentioned conventional polyimide resin is used as the binder, the adhesion and the strength of the resin have still been insufficient. Thus, the initial charge-discharge efficiency and the cycle performance have not been improved sufficiently. Therefore, there exists a need for improvements.
In view of the foregoing, it is a principal object of the present invention to provide a lithium secondary battery that can inhibit fractures of the binder resin and peeling-off of the negative electrode active material and the negative electrode current collector at the interfaces with the binder resin even when charge-discharge operations are performed repeatedly, so that the electron conductivity within the negative electrode can be prevented from deterioration and the cycle performance can be improved. It is another principal object of the invention to provide a method of manufacturing such a battery.
Accordingly, it is an object of the present invention to provide a lithium secondary battery that shows good cycle performance and also achieves high energy density even with the use of silicon particles and the like as the negative electrode active material, and to provide a method of manufacturing such a battery.